Method for monitoring an internal combustion engine

ABSTRACT

A method of monitoring an internal combustion engine by continuously computing pressure values of the cylinders from current parameters of the engines by using a mathematical model. At each point in time, there are both a computed pressure value and a measured pressure value, which are compared in order to provide information concerning the condition of a pressure sensor and to deactivate a faulty pressure sensor and use the computed pressure values for further operation of the engine.

BACKGROUND AND SUMMARY OF THE INVENTION

This application claims the priority of German Patent 199 27 846.6,filed Jun. 18, 1999, the disclosure of which is expressly incorporatedby reference herein.

The invention relates to a method of monitoring an internal-combustionengine in which pressure values of the cylinder space of theinternal-combustion engine are measured by means of a pressure sensorand the condition of the sensor is determined therefrom.

The proper functioning of an internal-combustion engine is monitoredduring the operation by an electronic engine control unit. Thus, it isknown, for example, from German Patent Document DE 41 27 950 A1 tomonitor the sensor detecting the internal pressure of the cylinders. Inthis case, the pressure values are checked as to whether they are withina tolerance band with variable limit values. The limit values, in turn,are a function of the loading of the internal-combustion engine.Pressure values outside the tolerance band are interpreted as beingfaulty. As a subsequent reaction, the corresponding cylinder is changedto idling. It is problematic that a short-term disturbance of the sensorsignal or the aging of the sensor will necessarily cause a cylinderswitch-off; that is, the internal-combustion engine will no longeroperate normally.

A diagnostic process for pressure sensors of an internal-combustionengine is known from European Patent Document EP 0 569 608 A1. In thisprocess, the pressure values in the compression cycle or in the upperdead center are compared with a reference value. In the case of apersistently faulty pressure sensor, a visual or acoustic faultindication is activated as a consequent reaction.

Based on the above-described prior art, it is an object of the inventionto ensure the normal operation of the internal-combustion engine as longas possible.

This object is achieved in that pressure values of the cylinder spaceare continuously computed from current parameters of theinternal-combustion engine by means of a mathematical model. At anypoint in time, a measured pressure value and a computed pressure valueare present isochronously. From the comparison of the computed pressurevalues and the measured pressure values, information is then obtainedconcerning the condition of the pressure sensor. With the detection of apermanently faulty pressure sensor, the latter is deactivated. Thefurther operation of the internal-combustion engine takes place on thebasis of the computed pressure values; that is, the computed pressurevalues are set as the relevant pressure values.

A permanently faulty pressure sensor is detected if, in a first step, adeviation is present outside a tolerance band consisting of a first andsecond limit value, and in a second step, the amount of the deviation isgreater than a third limit value. The deviation is determined from themeasured and computed pressure values.

If it is detected in the second step that the amount of the deviation issmaller than the third limit value, the measured pressure values arecorrected.

The method according to the invention therefore permits a normaloperation of the internal-combustion engine even if, for example, ashort-circuit of the sensor is present. It is another advantage that afaulty pressure sensor signal can be corrected by the reference values,for example, in the case of a zero-point fault or amplification fault ofthe pressure sensor.

The mathematical model for computing the pressure values of the cylinderspace is represented by a differences equation. The description of thethermodynamic relationships by way of a differences equation has theadvantage that the method is real-time-capable. This means that withrespect to a measured pressure value, a correspondingly computedpressure value is present at any point in time during the operation ofthe internal-combustion engine.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures illustrate a preferred embodiment of the invention.

FIG. 1 is a block diagram of an accumulator injection system of anengine;

FIG. 2 is a first program flow chart;

FIG. 3 is a second program flow chart;

FIG. 4 is a program flow chart of the computation of the differencesequation;

FIG. 5 is a graph of pressure and temperature of a cylinder.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an internal-combustion engine with amechanically controlled accumulator injection system (common rail). Itshows an internal-combustion engine 1 with a turbocharger and a chargeair cooler 2, an electronic engine control unit 10, a pump 4, ahigh-pressure accumulator (rail) 5, injectors 8 connected thereto and apressure control valve 6. As known, the pump 4 delivers the fuel from afuel tank 3 at a high pressure into the high-pressure accumulator 5. Thepressure level of the high-pressure accumulator 5 is detected by way ofa rail pressure sensor 7. Pipes with injectors 8 connected thereto foreach cylinder of the internal-combustion engine 1 branch off thehigh-pressure accumulator 5.

The electronic engine control unit 10 automatically controls thecondition of the internal-combustion engine 1. The following is shown asinput quantities of the electronic control unit 1): Pressure of thecylinder space plST(i) which is measured by means of pressure sensors 9;pressure (pCR(IST)) of the high-pressure accumulator 5; pressure level(pLL) of the turbocharger with the charge air cooler 2; and rotationalspeed (NMOT) of the internal-combustion engine 1. The additional inputquantities relevant for the operation of the internal-combustion engine1 are indicated by reference symbol E. Output quantities (A) of theelectronic engine control unit 10 are, for example, triggering signals(SB) for the injectors 8 and the triggering signal (pCR(MAX)) for thepressure control valve 6. The pressure level in the high-pressureaccumulator 5 is adjusted by way of the pressure control valve 6.

As illustrated in FIG. 1 by the two arrows, air is fed and exhaust gasesare discharged respectively to or from the turbocharger with the chargeair cooler 2.

FIG. 2 shows a program flow chart for computing pressure values of thecylinder space for all cylinders. The computing process is started afterthe conclusion of the initializing of the electronic engine control unit10. In step S1, a variable i is equal to 1. This variable corresponds tothe cylinder numbering. In step S2, it is checked whether theinternal-combustion engine 1 has started. If the test result isnegative, that is, the internal-combustion engine 1 has not yet beenstarted, there is a passing through a waiting loop in step S3. If theinternal-combustion engine 1 has been started, the cylinder pressurepBER(i) is computed in step S4. In the first program pass, this istherefore the pressure level of the first cylinder. The computingprocess by means of the mathematical model is explained in connectionwith FIG. 4. Then, in step S5, the measured cylinder pressure plST(i) isread in. In step S6, a deviation dp(i) is determined from the computedand the measured cylinder pressure. In step S7, it will then be checkedwhether this deviation dp(i) is within a tolerance band consisting of afirst limit value GW1 and a second limit value GW2. If the deviation iswithin the tolerance band, it is determined in step S8 that the pressuresensor is in the normal condition. Then, the program flow chart branchesto point B.

If the test result in step S7 is negative, it is additionally checked instep S9 whether the amount of the deviation dp(i) is smaller than athird limit value GW3. If this is so, the pressure sensor 9 is adjustedin step S12. This always occurs when an amplification or zero-pointfault of the pressure sensor 9 is present. In step S12, the measuredsignal plST(i) is therefore adapted to the reference value, thus to thecomputed cylinder pressure pBER(i). If it is determined in step S9 thatthe amount of the deviation dp(i) is larger than the third limit valueGW3, it will be determined in step S10 that the sensor has beenpermanently damaged. A permanent damage of the pressure sensor 9 exists,for example, in the event of a line interruption or a short circuit.Then, the pressure sensor is deactivated in step S11 and the computedpressure values pBER(i) are set as valid pressure values. For thefurther control and automatic control of the internal-combustion engine1, these computed pressure values pBER(i) will therefore be used; thatis, they represent a replacement function. In step S13, it is queriedwhether all cylinders have been tested. If this is so, the program flowchart branches to point A and starts again with step S1. If the testresult in step S13 is negative, that is, not all cylinders have yet beenqueried, by means of step S14, the pass variable i is increased by 1.Then the program flow chart is continued by means of step S2.

FIG. 3 shows a program flow chart for computing the pressure values ofthe cylinder interior when only one pressure sensor is used. Thisprogram flow chart will therefore be used when not all combustion spacesof the internal-combustion engine 1 are equipped with a pressure sensor9. The program flow chart corresponds to a reduced implementation ofFIG. 2, steps S1 to S11 corresponding to steps S2 to S12 of FIG. 2. Inthis case, the computing process is implemented analogously, so that theabove will apply.

FIG. 4 is a program flow chart for computing the internal cylinderpressure pBER(i) by means of the mathematical model. The program flowchart starts at point I when there is a first pass through thecomputation of the pressure; that is, after the starting operation ofthe internal-combustion engine 1. In step S1, a query point in time t isset to be equal to 0. In step S2, the input quantities of theinternal-combustion engine 1 will then be read in. Input quantitiesrequired for the computation are the pressure level pLL of theturbocharger with the charge air cooler 2, the fuel mass correspondingto the fuel volume to be injected, the fuel temperature and therotational speed nMOT or the crankshaft angle. In step S3, the heatcapacity with isochoric heating cV and the mass in the cylinder mZ areread in. The mass in the cylinder mZ can be determined in a tabulatedmanner or can be determined by means of a different method. Then, instep S4, a factor F is computed from the two values cV and mZ. In stepS5, the pressure level of the charge air cooler pLL is assigned to thecomputed pressure of the cylinder space pBER(t). This value is used asan initial value for the lower dead center of the charge cycle. Thus, byway of the mass mZ in the cylinder and the volume, the temperature ofthe gas is also known in step S6:

T(t)=pLL(VC+VH)/(R·mZ)

wherein

pLL charge air pressure VC dead volume VH volume cylinder R gas constantmZ mass in the cylinder

In step S7, the query point in time t is increased by 1. In step S8, aquantity A(t) is computed corresponding to the heat increase during thequery period t. Then, a quantity B(t) is computed in step S9corresponding to the volume change during the query period t. In stepS10, the temperature increase in the cylinder interior during the queryperiod is determined from the factor F, the quantities A(t) and B(t) aswell as the interior pressure pBer(t−1) previously computed (in?translator) a query cycle. The following applies:

ΔT=F·((A(t)−pBER(t−1)·B(t))·Δt

wherein

ΔT temperature increase Δt query period (t, t − 1) F factor f = (cV, mZ)A heat change in the cylinder (ΔOb/Δt) pBER pressure in the cylinder(computed) B volume change in the cylinder (ΔV/Δt))

wherein

cV heat capacity with isochoric heating mZ mass in the cylinder Ob heatof the burnt fuel V volume in the cylinder (VC, VC) t current querypoint in time t − 1 previous query point in time

The differences equation represents the thermodynamic relationships inthe cylinder interior. Here, the thermodynamic relationships to becomputed at high mathematical expenditures are illustrated by way ofthis simple differences equation. As a result, the advantage is achievedthat the process is real-time-capable. This means that a correspondinglycomputed value pBER(i) is present in an isochronous manner for eachmeasured pressure value of the cylinder interior plST(i). The processillustrated in steps S8 and S10 uses two points in time (t, t−1).Naturally, it is also possible to implement the process by way ofseveral time values.

In step S11, the temperature increase is added to the temperature valueof the preceding query point in time T(t−1). This results in the newtemperature value T(t). In step S12, the computed pressure value pBER(i)is then formed from the temperature T(t) by way of the general gasequation. Then the program flow chart branches to point III,corresponding to step S5 from FIG. 2 and step S4 from FIG. 3respectively. For the next pressure value pBER(i) to be computed, theprogram flow chart starts at point II.

FIG. 5 is a simplified representation of the course of the temperature Tor of the course of the pressure p in the cylinder space of theinternal-combustion engine 1 over the time t. These are indicated as asinusoidal curve. The operating point A on the curve is the result ofthe value p(0) measured at the point in time t(0). After the startingoperation, this is the pressure level pLL of the charge air cooler 2.This value is therefore the starting value for the computation process.The value T(0) results from the value p(0) and the gas equationcorresponding to step S6 of FIG. 4. At the point in time t(1), thetemperature increase ΔT is computed according to the computation processof FIG. 4. This results in point B on the curve with the ordinate valuesT(1) or p(1). At the point in time t(2), the new temperature increase iscomputed. This results in the point C on the curve. The furthercomputation of the temperature and pressure values respectively takesplace in the same manner.

The use of a differences equation instead of a exponent equation for thethermodynamic operations has the advantage that the model is real-timecapable. In that a computed value pBER(i) is available at any point intime for the measured internal cylinder pressure plST(i), the pressuresensor 9 can be monitored and can be adjusted in the event of a drift.Likewise, a permanently damaged pressure sensor is clearly recognized.In this event, the further operation of the internal-combustion engineis ensured on the basis of the computed pressure values pBER(i).

What is claimed is:
 1. A method of monitoring an internal combustionengine comprising the steps of: measuring pressure values of cylindersof the internal combustion engine by means of a pressure sensor;monitoring the condition of the pressure sensor by means of the measuredpressure values; continuously computing, by means of a mathematicalmodel, computed pressure values of the cylinder as a function of currentparameters of the internal combustion engine whereby, at any point intime, a measured pressure value and a computed pressure value areisochronously present, resulting from a comparison of the measuredpressure value and the computed pressure value; checking the pressuresensor; deactivating the pressure sensor when a permanently faultypressure sensor is detected; and setting the computed pressure values asrelevant pressure values and wherein continued operation of the internalcombustion engine occurs on the basis of the computed pressure values.2. The method according to claim 1, wherein the step of detecting apermanently faulty pressure sensor includes detecting of a deviation ofmeasured values outside a tolerance band, wherein said tolerance bandhas a first limit value and a second limit value, and wherein the amountof deviation is greater than a third limit value and wherein thedeviation is determined from the measured pressure values and thecomputed pressure values.
 3. The method according to claim 2, wherein,when the deviation is within the tolerance band, a normal condition ofthe pressure sensor is determined.
 4. The method according to claim 2,further comprising the step of correcting the measured pressure valuewhen the amount of deviation is smaller than the third limit value. 5.The method according to claim 1, wherein the mathematical model isdescribed by a difference equation in the form ofΔT=F.((A(t)−pBER(t−1).B(t)).Δt wherein ΔT temperature increase Δtperiod(t, t− 1) F factor f = (cV, mZ) A heat change in the cylinder(ΔOb/Δt) pBER pressure in the cylinder (computed) B volume change I thecylinder (ΔV/Δt)

wherein cV heat capacity with isochoric heating mZ mass in the cylinderOb heat of the burnt fuel V volume in the cylinder t current query pointin time t − 1 previous query point in time.


6. The method according to claim 5, further comprising the step of usingthe pressure level of air compressed by a turbo-charger with a chargeair cooler as the starting quantity for the mathematical model.
 7. Amethod of monitoring an internal combustion engine in which measuredpressure values (pIST(i), I=1,2 . . . n) of the cylinder space of theinternal combustion engine are measured by means of a pressure sensorand the condition of the pressure sensor is monitored by means of themeasured pressure values (p(IST(i)), wherein, from current parameters ofthe internal combustion engine, computed pressure values (pBER(i), I−1,2. . . n) of the cylinder space are continuously computed by means of amathematical model, so that, at any point in time, a measured pressurevalue (pIST(i)) and a computed pressure value (pBER(i)) areisochoronously present, by means of a comparison of the measuredpressure value (pIST(i)) and the computed pressure value (pBER(i)), thepressure sensor is checked, with the detection of a permanently faultypressure sensor, the latter being deactivated, the computed pressurevalues (pBER(i)) being set as relevant pressure values, and thecontinued operation of the internal combustion engine taking place onthe basis of the computed pressure values (pBER(i)).